He can, that's why he's crying
They clearly only have him crying as crappy plot development to move the relationship in the story forward. He wouldn't have been secretly meeting the grandmother if he really felt guilt and empathy for what Hyesung went through. He's crying because he feels bad for himself
It would be long, but I do feel the shame. I don’t really enjoy how they’re twisting such a sensitive subject just for plot development.
As for Byul… it’s complicated. We, the readers, have a completely different point of view. We know about Hyesung’s past. We know what he’s been through — the assault, the rape, his struggles. We also know the deep impact of being abandoned by his mother. We know everything about Hyesung.
But Byul doesn’t. He thinks he does, but he really doesn’t know better. He barely knows the general outline; the rest are just assumptions based on what he’s observed. Like how he observed how loving and talented Hyesung is, and assumed Hyesung has always been like this.
Another thing to remember is that Byul is still a child — just a teenager. He makes mistakes. And we know he didn’t mean what he said to Hyesung.
What I also want to add is that Byul is an empathetic person — maybe too empathetic. And unfortunately, in this case, his empathy is directed more toward his grandmother than his mother. Why? Because the grandmother told her version of the truth — the version that benefits her. Meanwhile, Hyesung stays silent. He doesn’t talk about what really happened, and he has everything to do so. So we’re left in this uncomfortable space where the grandmother was a victim but had the time to heal, but she still hurt Hyesung deeply — and continues to.
And now we have Byul, who thinks he knows what happened — but he doesn’t. A Byul who stays in contact with the grandmother and excuses it, partly because his own father does the same. A Byul who ends up minimizing what Hyesung went through, because he believes he already knows all he needs to know.
So he can have empathy for his grandmother but he can't have empathy for Hyesung who centered his entire life around raising Byul?